Loud-speaking telephone systems



March 17, 1959 R. F. LAURENCE LOUD-SPEAKING TELEPHONE SYSTEMS 2 Sheets-Sheet 1 Filed Feb. 15, 1955 ,ZFLaa/zvezzce Mrch 17, 1959 R. F. LAURENCE 2,878,314

LOUD-SPEAKING TELEPHONE SYSTEMS Filed Feb. 15, 1955 2 sheets-sheet V2 N. gztmw N TV* FS.

g; ww--In rum Y lx-A 'F Q ,q *H H; n@ Q l A v United States Patent() K o i i i 2,878,314 f LOUD-SPEAKING TELEPHONE SYSTEMS Roger F. Laurence, Guildford, England, assignor` to Winston Electronics Limited,lHarnp`ton, England IThis invention relates to a loud-speaking telephone system for two, three `or four wire lines of the kind in lwhich in order that the loop gain may be less than l unity for any loop and `oscillations shall not occur,` a proportion of thesignal in each channel is rectiedand Ithe resultant uctuating direct signals are compared by a muting` control circuit andthe channel producingthe smaller lsignal is muted, receivefcliannel is muted at any time in `accordance withl the signal conditions in the channel. The direct voltage is normally developed from the audio-frequency output by a rcctiier system with a condenser input` filter having a pick-up time of three milliseconds or less` to avoid clipping of `initial speech syllables.

The discharge time of the condenser input iilter is made longer than thepick-up time e. `g. by 10() milliseconds in thecase of the `rectifier associated with the unmuted channel. By pick-up time is meantthe time from the moment whenthe reservoir condenser starts charging due to a signal applied to the rectifier to the time when the voltage across the condenser reaches l/e of the,` peak value of the applied signal.

so that either the transmiuor x o telephone made accordingI t'otthe invention,

According to the invention the ratio of signal amplitude from the microphone or line before muting to directv control voltage or current produced by it at the muting control circuit is made` greater in the case of the muted channel than in the case` of the `unmuted channel. Conveniently,l one channel is muted about one millisecond before the lother is released during changeover. WTO` overcome the diiiculties caused by the proximity of the microphone to the loud-speaker as compared to the operator the proportion lof the `signals from the microphone fed `to the rectifier associated with it is increasedwhen the station is transmitting.

Similarly, toreduce the difficulties caused atjtlieline connection of two wire systems and between rrnote loud-speaker fand microphone of any system, the proportionv of 'the incoming signals fed to the associated rectitieris reduced when the station is transmitting and increased when the station is receiving. The `muting by coupling control system must not introduce any large transients in the `equipment `when it operates.

i The muting control circuit should have backlash `so that the output of the signahrectier associated with the muted channel must become appreciably larger than 'the output of that associated with the released channel, before the muting control circuit mutes the released channel and releases the muted channel.

When used on twowire lines the equipment yshould be connected to the line by a transformer in the loutput circuit of a valve amplifier stage. A similar valve ampliiier stage and transformer should be supplied, in parallel with `,the first, with signals originating from the microphone. The outputwindings of the transformers should be connected inphase opposition to the receivinglainpliiier.`

.i phone -muting unit Als/IMU)` to the ,this gives the equipment.

ice

. With thisfarragement signals from themicrophone are cancelled at the input to the receiving ampliiier while line signals are fed directly. The mnting` system should not completely suppress the signals supplied to the distant operator from the local operaton'so as to avoid the dead feeling which l"lhecontrol rectiers should` be supplied by separate amplifiers and not` by amplifiers in the` main speech channels, to` avoid distortionof the speech received at tlietother end. l

`Alternatively, the high `frequency components of the speech signals `may be emphasized and after the amplifier stage ,which drives the rectitier, and `which is also in the `speech channel, the relative amplitude of the high frequency components i.

M o o e. the ratio` of the voltage of the signal from the microphone or liner (before muting) to the `direct voltage produced by it across the reservoir condenser associated with fthe rectitiers may be subsequently reduced by means of a iilter network, so that the effect of the distortion introduced `by the loading of the rectifier `system is minimised.` f i f The invention will bel further `described with reference to embodiments shown in the` accompanying drawings. Figure `1 is a` schematic of a two-wire loud-speaking and shows oneeomplete station. o ,o 1 1 i `Figure 2 is afcircuit diagram1 of the arrangement-of Figure 1. i Y V l Figure 3 shows a four` wire loud-speaking telephone amplifier in which a double central amplifier is `used with stations consisting merely of microphones and loudspeakers, connected by long leads to ithey central am-l plifier. M1 and LSZ are at one station and M2 and LS1 at the other. 'i

Referring to Figures 'l and 2 signals from the microf phone MIC are amplified by .the f microphone amplier MA, comprising valves V1 and V2, and passed via the microphone muting unit MMU, consisting of transformers T2 and T3" andcontact a1, tothe line unit LUcom prising valves VSand V4. Incoming signals are fed to the line amplitier LA, comprising valves V5 and V6, and via the line muting unit LMU,.transformer winding T6, contact a1, to the lond-speakerLS.` o

If the localoperator `is talkingv the contact a1 connects leads T andU and puits` the full signal fon` the line. VLeads V and W are connected through a resistance and only weaksignals reachthe loud-speaker. If the distant operator is talking contact a1. connects leads V and W and puts the full signal` into the loud-speaker, while leads T and U are 'connected through a resistance and only weak microphone signals reach the line. AIf the local operator i's` talking the signals from the centre point of resistance R6, R7 added .tothe output of transformer T3 are ,fed` over` lead Y` (auxiliary microv microphone rectifier" MR, consisting of rectifiers M111 and MR2, via the amplifier valve V7 and cathode follower valve V8.

If the distant operator starts talkinghhis signals appear across resistance R25 (auxiliary l"line muting unit ALMU) in the output valve 6 and these added to the small output from transformer are fed over lead Z to the cathode follower valve V9 and line rectifier` LR comprising rectiiers MRS and MR4. If his signals are large `enough the polarity oftthe potential on the grid ofvalve V10` will be reversed and the Schmitt trtigger tive yto the speech `signals in thech'annels, i. "'e., 'those' muted channel. `If the localoperato'r vno'ifv talks "s'u'i vciently loudlythe signal over lead'Y again exceeds that Aover lead Zand the relays A/jl, `B/l willr'be -restored to the condition giving the local 'operator vthe channel.

The pick-up rtime of about `3 y vachieved by nsinga full-wave rectifier and by Vusing a 'cathode vfollovv'e'r `as a low impedance signal source to `promote v'rapid charging of thecondensers Cof the condenser input Vfilter. p

The `relay contact a` 1 niutes one channel one milli- `second before releasing'the other.

lLeads Y and Z carry ysignalsfrom their respective y'speechcliann'els 'to lassociated rectiers at all times.

Lead Y is associatedwth the 'microphone 'in go' channel and lead Z withl the lineror"returrichannel.

The 'signalsfrom transformer Ttolead 'Y and transformer T7 to lead Z y transit or receivechannel xs 'muted 'by the'short circuit. ing of TU 'or VW. l I

Contacts a1 and b1 disturb nodirect crrent'conditiris `and so Acreate 1no largetransients. Y u p The'rely cdritact'bl 'switches a relatively vIow'resistance across theV condensers C of the rectiers'"a`s'sociated `tivith'the inte'd channel. u The 'piek-uptime is proportional Ato the capacity 'of the condenser and theforward resistance `of the rectiers (assuming a low impedance A. generator). denser and 'the resistance of the shunt resistance -or the These quantities are chosen to 'give the desired result.

n The Schmitt trigger circuit "or MCU comprising valves V11 and direct Ycoupled amplifier valve V has backlash jIn this trigger circuit a certainv stablest'ate may be induced ,by the 'application of aY suicientlypositive' volt- V12, driven gaby the Jewv "is the backlash 'voltage Aof tlief'cicuit. l

and V4 have lth'eir grids 'supplied the mlcrophoneamplifier and thetransformers 'in theirano'de circuits have thesecondar'ies connectedin series opposition.`

Resistances R17 v'and R18 permits'ome signals to` pass inthe muted channel.

Resistances follower valves'VS and V9, th'us system from the speech system.

For use on three wire lines the line unit comprising valves vV3 and V4is dispensed V4with land lthe go wire is connectedto lead X. yThe common-'wire is connected to "earth andthe return vvire'to the grid'of valve V5.

"muting control unit Suitable isolating condensers and vtransformers can 'be interposed. v

`For usefon-fourwire'lines -the lower secondary wind-l ing of transformer T3 is connected to the go pair va 'an isolating condenser `if desired, 'the return "pair 'feeds the grid circuit of valve V5 via a condenser and transformer.

Referring to Figure 3 station A consists of a carbon microphone M1 and a loud speaker LS1. Station B consists of microphone M2 and a loud speaker LSZ.

The signals from the microphone M1 are amplified by valves V10, and V20 and V30 and those 'from lmicrophone M2 by valves V40, V50 and V60.

They are then extended over rectifiers 'MR and 'condenser input lters formed by capacitors C and resistances R to -the input of'valve V70.

If the signals `from valve V30 are larger than those from valve V60 the grid of valve V70 is driven far enough in a positive direction relative to the cathode to cause the Schmitt trigger circuit connected to the anode to operate `the relays A/l, B/ 1 and C/ 1. This `increases the direct voltage due to the signals from valve V30 relative to that due to the signals from valve V60, (at contacts b l, c1), connects loud speaker LS2, l-and disconnects `loud speaker LS1 (at contact al).

To avoid distortion due to the loading of the rectifier systems on valves V30 and V60, the microphones used are chosen for a rising high frequency characteristic and then the condensers in the anode circuits of valves V30 and V60 give adropping high frequency characteristic at ythese t points.

The use of a microphone with phone and a speaker and in which, in order that the loop producing the smaller signal transmit or the receive channel is muted at any time, the muting control circuit being connected to be operated by direct control signals A2; The combination of claim 1 including the additional switch means operable for reducing the proportion of the signal fed tothe associated rectifier system when a v station is transmitting.

3. In a loud-speaking telephone system of the kind employing `multiple channels each connecting a microphone and a speaker and in which, in order that the pendent of the channels carrying speech signals from one operator to another, nttenuator means connected `with each of the channels, and switch means controlled by the muting control circuit for bringing into effect the attenuator means associated with the muted channel for increasing the ratio of signal amplitude to direct control signal produced therefrom in the case of the muted channel as compared with the unmuted channel.

The combination of claim 3, including means for supplying a signal, a rectifier system amplifier in which 10 6 the high frequency components of the signal are 'emphaszed and a filter network for attenuating the signal after the amplifier stage whereby the effect of distortion produced by the loading of the rectifier system is mini 5 mised.

References Cited in the tile of this patent UNITED STATES PATENTS 2,702,319 Ryall Feb. 15. 1955 

